Golf ball

ABSTRACT

A golf ball which improves adherence of a coating to a cover and which improves spin retention without sacrificing the other properties of the coating. The golf ball has a core, a cover of at least one layer covering the core and a coating formed on an outer surface of the cover, wherein a thickness of the coating is 25 to 125 μm, a 50%-modulus of the coating is 5 to 50 MPa, and a value R expressed by R=PL/CL/1000 is 0.01 to 0.5, where CL (mm) represents a thickness of an outermost layer of the cover and PL (μm) represents a thickness of the coating.

BACKGROUND OF THE INVENTION

[0001] 1. Field of the Invention

[0002] The present invention relates to a golf ball having a high spin retention, that is, a high ratio of a spin rate under a wet condition to a spin rate under a dry condition.

[0003] 2. Description of the Background Art

[0004] A surface of a golf ball is generally covered with a coating to give a gloss in appearance. As a golf ball is used in a severe environment, it is very difficult to retain a glossy appearance for a long period, and demanded levels of various properties of the coating are inevitably high. Some of the properties demanded for the coating are, for example, flexibility to prevent cracking even when the ball is deformed to a large extent by hitting with a driver or the like, abrasion resistance to prevent damage by large friction caused by an iron shot, wear resistance for a bunker shot, and durability to prevent peeling of the coating by hitting with various clubs or by using the ball in a rainy day.

[0005] In Japanese Patent Laying-Open No. 2001-17574, for example, the abrasion resistance is improved by setting tensile strength of a cover material and a coating to approximate values to enhance adherence of the coating to the cover. In Japanese Patent Laying-Open No. 2000-176047, in addition, the abrasion resistance and wear resistance are enhanced by making a thickness of a coating as small as 5-20 μm.

[0006] Conventionally, golf balls with balata covers are generally and widely used by low handicap and professional golfers by virtue of their superior hit feeling and controllability. Recently, various soft covers have been proposed to replace the balata covers, however, because of their complicated fabrication process and poor cut resistance.

[0007] In a golf ball having improved controllability proposed in Japanese Patent Laying-Open No. 10-179802, a basic resin of a cover is formed having as a main component a heated mixture of two components, that is, an ionomer resin and a styrene-butadiene-styrene block copolymer including a polybutadiene block containing an epoxy group or a styrene-isoprene-styrene block copolymer including a polyisoprene block containing an epoxy group. A composition for the cover has a flexural modulus of 50-300 MPa and a Shore D hardness of 40-60.

[0008] In a golf ball having improved controllability proposed in GB 2311530, a basic resin of a cover is formed having as a main component a heated mixture of three components, that is, an ionomer resin, an acid-modified thermoplastic elastomer or a thermoplastic elastomer having an OH group at the end of the polymer chain, and a styrene-butadiene-styrene block copolymer including a polybutadiene block containing an epoxy group or a styrene-isoprene-styrene block copolymer including a polyisoprene block containing an epoxy group. A composition for the cover has a flexural modulus of 50-300 MPa and a Shore D hardness of 40-60.

[0009] In Japanese Patent Laying-Open No. 2001-137386, in addition, a golf ball having a high spin retention, that is, a high ratio of a spin rate under a wet condition to a spin rate under a dry condition is proposed, wherein a loss tangent (tanδ) at −10° C. in dynamic viscoelasticity of a cover material is set to 0.15-0.70.

[0010] All of these prior arts improve the spin retention in terms of a cover material, not in terms of a coating of a golf ball.

SUMMARY OF THE INVENTION

[0011] The present invention provides a golf ball which improves adherence of a coating to a cover by respectively setting a thickness of the coating, a 50%-modulus and a ratio of a thickness of the cover to that of the coating of the golf ball to certain ranges, and which improves a spin retention without sacrificing needed properties of the coating.

[0012] A golf ball according to the present invention has a core, a cover of at least one layer covering the core and a coating formed on an outer surface of the cover, wherein a thickness of the coating is 25 to 125 μm, a 50%-modulus of the coating is 5 to 50 MPa, and a value R expressed by the following equation (1) is 0.01 to 0.5:

R=PL/CL/1000   (1)

[0013] where CL (mm) represents a thickness of an outermost layer of the cover and PL (μm) represents a thickness of the coating.

[0014] The coating is preferably formed with polyurethane paint including polyol and polyisocyanate. In addition, it is more preferable that an equivalent ratio of trifunctional polyol to bifunctional polyol included in the aforementioned polyol (trifunctional polyol/bifunctional polyol) is 1.0-2.5. In the present invention, when the cover is formed with a plurality of layers, the outermost layer of the cover preferably has thickness CL (mm) of 0.5 to 2.5.

[0015] The foregoing and other objects, features, aspects and advantages of the present invention will become more apparent from the following detailed description of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

[0016] A golf ball according to the present invention is characterized in that, a thickness of a coating is 25 to 125 μm, a 50%-modulus of the coating is 5 to 50 MPa, and a value R expressed by the following equation (1) is 0.01 to 0.5:

R=PL/CL/1000   (1)

[0017] where CL (mm) represents a thickness of an outermost layer of the cover and PL (μm) represents a thickness of the coating.

[0018] One of the most important properties of the coating is peeling resistance, that is, a property of the coating of not peeling off a cover. A golf ball is subject to substantial deformation due to an external force applied by hitting with a golf club. When a golfer hits a golf ball with a head speed of about 35-50 m/sec, for example, a body of the golf ball (portions other than a golf ball coating) deforms by about 40%. Therefore, if adherence of the coating to the golf ball body is insufficient or the coating is too hard and elasticity thereof is insufficient, the coating cannot follow the deformation of the ball body, which results in cracking of the coating or peeling of the coating away from the cover.

[0019] The peeling resistance of the coating is becoming especially important in recent years where a solid golf ball using a cover including an ionomer resin as a base material is becoming mainstream. This is because, as the cover made of the ionomer resin has high strength and breaking of the cover rarely occurs, a lifetime of the golf ball is determined by peeling of the coating away from the cover.

[0020] As a result of the research by the inventors on a relation between the peeling resistance and the thickness and 50%-modulus of the coating, it is shown that the thicker the coating is, the less probable the peeling of the coating becomes. That is, as the coating becomes thicker, a ratio of the thickness of the coating to the amount of deformation of the golf ball increases, and the peeling of the coating becomes less probable. In addition, we found that the spin retention becomes higher by setting the ratio (R) of the thickness of the coating (PL) to the thickness of the outermost layer of the cover (CL) to 0.01 to 0.5, and setting the 50% modulus of the coating to 5-50 MPa.

[0021] <Coating>

[0022] The golf ball according to the present invention is characterized in that, the thickness of the coating covering the outermost layer of the cover (PL) is 25 μm to 125 μm. In a conventional golf ball, a coating as thin as 15-30 μm is used. According to the present invention, the peeling resistance can be improved by making the thickness of the coating (PL) larger so that the coating can follow the deformation of the golf ball at hitting. It is preferable that the thickness of the coating (PL) is 35-100 μm, especially 40-80 μm. The thickness of the coating is defined as an average value of the thicknesses at 20 points on a surface of the golf ball; 10 points on the dimple portions and 10 points on the land portions.

[0023] In the present invention, the 50%-modulus of the coating is set to 5-50 MPa. If the 50%-modulus is lower than 5 MPa, the coating of the ball is damaged by friction due to hitting with a fairway wood or an iron club, and wearing of the coating occurs with an iron club and sand during a bunker shot. If the 50%-modulus is higher than 50 MPa, on the other hand, the coating easily cracks for lack of flexibility, which degrades the appearance. In addition, the coating peels off the cover due to the cracking of the coating. The 50%-modulus of the coating is more preferably 10-40 MPa, and further preferably 15-30 MPa. The 50%-modulus of the conventional coating is about 60 MPa.

[0024] Variety of material, degree of curing, molecular weight and the like of the paint may be adjusted to set the 50%-modulus of the coating within the above-mentioned range. Examples of the paint that can be employed in the present invention include, but are not limited to, alkyd resin paint, phenol resin paint, amino resin paint, amino-alkyd resin paint, aminoacrylic resin paint, unsaturated polyester resin paint, epoxy resin paint, polyurethane paint, chlorinated rubber paint, acrylic resin paint, vinyl resin paint, and fluororesin paint. Among these, polyurethane paint can suitably be used because of its high glossiness and the like.

[0025] Polyurethane paint forms a coating having urethane bonds by a curing reaction of polyol and polyisocyanate. Examples of polyisocyanate include aliphatic diisocyanate, alicyclic diisocyanate, aromatic diisocyanate, and aromatic-aliphatic diisocyanate such as hexamethylenediisocyanate, xylylenediisocyanate, hydrogenated xylylenediisocyanate, isophoronediisocyanate, tetramethyl-xylylenediisocyanate, hydrogenated diphenylmethanediisocyanate, and tolylenediisocyanate. Single polyisocyanate as such may be used, or two or more kinds may be mixed for use. Among these, aliphatic diisocyanate and alicyclic diisocyanate can suitably be used because of their high weatherability and non-yellowing property.

[0026] Diol and triol having low molecular weights and polymer polyol can be used as the above-mentioned polyol. Examples of the low molecular weight diol include ethylene glycol, diethylene glycol, triethylene glycol, butylene glycol, 1,3-butanediol, 1,4-butanediol, neopentyl glycol, and 1,6-hexanediol. Examples of the low molecular weight triol include glycerine, trimethylol propane and hexanetriol. Examples of the above-mentioned polymer polyol include polyether polyol such as polyethylene glycol, polypropylene glycol and polytetramethylene glycol, polyester polyol such as polyethylene adipate and poly-ε-caprolactone, acrylic polyol formed by properly introducing hydroxyl group into an acrylic polymer, polycarbonate diol synthesized using cyclic diol, and urethane polyol formed by reacting a diisocyanate compound with diol and triol to form urethane bonds.

[0027] When the coating is formed using polyurethane paint, the modulus of the coating can be adjusted with the molecular weight of the polyol and the number of hydroxyl groups. If the molecular weight of the polyol is made higher, for example, the modulus of the coating becomes lower because soft segments increase. If the molecular weight of the polyol is made lower, on the other hand, the modulus of the coating becomes higher because soft segments decrease. If a larger amount of bifunctional diol is included, the modulus of the coating becomes lower because diol binds to isocyanate linearly. If a larger amount of trifunctional triol is included, while the modulus of the coating becomes higher because triol binds to isocyanate in a three-dimensional manner, flexibility decreases and cracking or peeling easily occurs. Thus, the 50%-modulus of the coating can be adjusted to 5-50 MPa by setting an equivalent ratio of trifunctional polyol to bifunctional polyol included in the polyol (trifunctional polyol/bifunctional polyol) to 1.0-2.5.

[0028] In addition, it is preferable that an equivalent ratio of isocyanate groups in polyisocyanate to hydroxyl groups in polyol (NCO groups/OH groups) is 0.9-1.5. When the equivalent ratio is lower than 0.9, the coating will still be sticky after a curing process, and dusts or contaminants may easily adhere thereto. When the equivalent ratio is higher than 1.5, on the other hand, some of the isocyanate groups will remain, which may react with water and generate CO₂ to cause bubbles within the coating.

[0029] In a composition of the coating according to the present invention, additives such as a silicon-based slip agent, a leveling agent, a viscosity adjustment agent, a fluorescent brightening agent, a blocking inhibitor, a curing catalyst, and a pigment may be included as needed.

[0030] <Relation Between Thickness of Outermost Layer of Cover and Thickness of Coating>

[0031] The golf ball according to the present invention is also characterized in that, a value R expressed by the equation R=PL/CL/1000 is 0.01-0.5. When the value R is lower than 0.01, durability of the golf ball decreases. When the value R is higher than 0.5, on the other hand, a repulsive force of the golf ball decreases, and thus a carry thereof decreases. It is more preferable that the value R is 0.0125-0.25. In a conventional golf ball, the durability is improved by making a coating harder and thinner, and the value R thereof is 0.002 to less than 0.01.

[0032] Herein, the thickness of the outermost layer of the cover (CL) is 0.25-2.5 mm, more preferably 0.5-2.0 mm. When the thickness of the outermost layer of the cover is smaller than 0.25 mm, durability of the golf ball decreases. When the thickness of the outermost layer of the cover is larger than 2.5 mm, a repulsive force of the golf ball decreases, and thus a carry thereof decreases. The cover of the golf ball according to the present invention can be formed with a plurality of layers. In this situation, a total thickness of the cover (cl) is 0.4-5.0 mm, preferably 0.5-3.0 mm. A ratio of the coating thickness (PL) to the cover thickness (cl) (PL/cl) is preferably 0.05-0.4. When the cover is formed with a single layer, the thickness of the outermost layer of the cover equals the whole thickness of the cover.

[0033] It is preferable that, a flexural modulus of the outermost layer of the cover is 2000-50000 N/cm². By setting the value within this range, adherence of the coating thereto can further be improved, and the peeling of the coating can decrease.

[0034] <Cover>

[0035] In the present invention, a material of the cover is not specifically limited. An ionomer resin can be used, such as a binary copolymer of α-olefin and α,β-unsaturated carboxylic acid having a carbon number of 3-8, that is produced by neutralizing at least a part of the carboxyl group with metal ion. Also listed as the ionomer resin is a ternary copolymer of α-olefin, α,β-unsaturated carboxylic acid having a carbon number of 3-8 and α,β-unsaturated carboxylate having a carbon number of 2-22, that is produced by neutralizing at least a part of the carboxyl group with metal ion.

[0036] As the above-mentioned α-olefin, ethylene, propylene, 1-butene, 1-pentene, for example, are employed, wherein ethylene is particularly preferable. As the above-mentioned α, β-unsaturated carboxylic acid having a carbon number of 3-8, acrylic acid, methacrylic acid, fumaric acid, maleic acid, crotonic acid, for example, are employed, wherein acrylic acid and methacrylic acid are particularly preferable. As the above-mentioned α, β-unsaturated carboxylate, methyl ester, ethyl ester, propyl ester, n-butyl ester, isobutyl ester or the like of acrylic acid, methacrylic acid, fumaric acid, maleic acid and the like, for example, are employed, wherein acrylic ester and methacrylic ester are particularly preferable.

[0037] Examples of the metal ion for neutralizing at least a part of the carboxyl group in the copolymer of α-olefin and α,β-unsaturated carboxylic acid or the ternary copolymer of α-olefin, α,β-unsaturated carboxylic acid and α,β-unsaturated carboxylate as described above include sodium ion, lithium ion, zinc ion, magnesium ion, potassium ion, and the like.

[0038] The specific examples of the ionomer resin as described above are those which are commercially available from Du Pont-Mitsui Polychemicals Co., Ltd. under the trade names of Hi-Milan 1555 (Na), Hi-Milan 1557 (Zn), Hi-Milan 1605 (Na), Hi-Milan 1706 (Zn), Hi-Milan 1707 (Na), Hi-Milan AM7318 (Na), Hi-Milan AM7315 (Zn), Hi-Milan AM7317 (Zn), Hi-Milan AM7311 (Mg), Hi-Milan MK7320 (K), Hi-Milan 1856 (Na), Hi-Milan 1855 (Zn), Hi-Milan AM7316 (Zn), and the like.

[0039] The ionomer resins commercially available from Du Pont further include Surlyn 8945 (Na), Surlyn 8940 (Na), Surlyn 9910 (Zn), Surlyn 9945 (Zn), Surlyn 7930 (Li), Surlyn 7940 (Li), Surlyn AD8265 (Na), Surlyn AD8269 (Na), and the like.

[0040] Ionomer resins commercially available from ExxonMobil Chemical Company include lotek 7010 (Zn), lotek 8000 (Na) and the like. It is noted that, Na, Zn, K, Li, and Mg shown in the brackets after the trade names of the ionomer resins described above indicate the neutralizing metal ion species thereof. In the present invention, the ionomer resin used as the base resin of the cover may be a mixture of two or more kinds of those illustrated above, or a mixture of two or more kinds of the ionomer resins neutralized with a bivalent metal ion and the ionomer resin neutralized with a monovalent metal ion as illustrated above.

[0041] For the cover according to the present invention, one or more kinds of thermoplastic elastomer having a rubber component can be mixed with the ionomer resin for use. The thermoplastic elastomer having a rubber component is a block copolymer having a conjugated diene component such as a butadiene block or an isoprene block. As the conjugated diene compound, for example, one or two or more kinds can be selected from butadiene, isoprene, 1,3-pentadiene, and 2,3-dimethyl-1,3-butadiene. Among these, butadiene, isoprene and the combination thereof are particularly preferable.

[0042] Specific examples of the thermoplastic elastomer include a styrene-butadiene-styrene block copolymer (SBS structure), a styrene-ethylene-butylene-styrene block copolymer formed by hydrogenating the double bonded portion of butadiene thereof (SEBS structure), a styrene-isoprene-styrene block copolymer (SIS structure), a styrene-ethylene-propylene-styrene block copolymer formed by hydrogenating the double bonded portion of isoprene thereof (SEPS structure), and modified products thereof. An example of the block copolymer having the SEBS or SEPS structure with a hydroxyl group added to the end portion thereof is commercially available from Kuraray Co. under the trade name of HG-252.

[0043] A part of the copolymer of the SBS, SEBS, SIS, or SEPS structure described above may be modified with an epoxy group. The styrene-butadiene-styrene block copolymer (SBS structure) having a polybutadiene block containing an epoxy group, for example, may be a block copolymer having polystyrene at both terminal ends and having polybutadiene containing an epoxy group as an intermediate portion of the chain, wherein a portion or all of the double bonds of the polybutadiene portion is hydrogenated.

[0044] The styrene-isoprene-styrene block copolymer (SIS structure) having a polyisoprene block containing an epoxy group may be a block copolymer having polystyrene at both terminal ends and having polyisoprene containing an epoxy group as an intermediate portion of the chain, wherein a portion or all of the double bonds of the polyisoprene portion is hydrogenated. A specific example of a diene block copolymer modified with an epoxy group includes Epofriend from Daicel Chemical Industries, Ltd.

[0045] Other thermoplastic elastomers include a polyamide thermoplastic elastomer commercially available from Toray Industries, Ltd. under the trade name of Pebax, a polyester thermoplastic elastomer commercially available from Du Pont-Toray Co., Ltd. under the trade name of Hytrel, a polyurethane thermoplastic elastomer commercially available from BASF Polyurethane Elastomers, Ltd. under the trade name of Elastollan, and the like.

[0046] In the present invention, the cover composition may properly be blended with additives such as a pigment, an antioxidant, a plasticizer, a dispersant, and an ultraviolet absorber as necessary.

[0047] <Cover Formation>

[0048] To form the cover as described above on a surface of a core, first the cover composition is prepared by mixing the base resin of the cover with additives such as a pigment, a dispersant, an antioxidant, an ultraviolet absorber, and a photo-stabilizer as needed, and then heating the mixture with mixing for 0.5-15 minutes at 150-250° C. using an internal mixer such as a twin-shaft kneading-type extruder, a Banbury mixer or a kneader. The cover composition may directly cover the core, or it may cover the core with another layer such as an intermediate layer interposed therebetween.

[0049] The cover may cover the core with a conventionally known method which is not specifically limited. As an example, the cover composition may be formed into a half-shell shape, and a pair of the half-shells may cover the core to form a sphere with heating at 130-170° C. for 1-15 minutes. Alternatively, the cover may be formed by injection-molding the cover composition onto the core. During the formation, the cover is formed with dimples on a surface of the ball.

[0050] In the present invention, the coating as described above is formed on the outer surface of the cover. A conventionally known application method can be adopted to form the coating on the cover. A spray gun or an electrostatic painting, for example, can be employed. When two-component mixture-type paint including the above-described polyol and isocyanate is to be applied, the two components are previously mixed or mixed just before the application. When the paint is dried after the application, it is generally preferred that the drying temperature is lower than 50° C. A drying period, which is generally 0.5-5 hours, may suitably be determined depending on the kind of the coating resin or the like.

[0051] The golf ball according to the present invention is applied to a golf ball having any structure which is formed with a core, a cover covering a surface of the core and a coating applied on a surface of the cover. A thread layer or an intermediate layer, for example, can also be formed between the core and the cover.

[0052] <Core>

[0053] In the present invention, the core is formed with a vulcanizate of a rubber composition formed by blending base rubber with organic peroxide as a cross-linker and α,β-unsaturated carboxylic acid or metal salt thereof as a co-crosslinker. Hardness of the core is adjusted by changing the type of the base rubber or a degree of cross-linking.

[0054] The base rubber used in the rubber composition for the core may be diene rubber conventionally used in the core of the solid golf ball, for example natural rubber or synthetic rubber. Examples of the synthetic rubber include an ethylene-propylene-diene ternary copolymer (EPDM), butadiene rubber (BR), isoprene rubber (IR), styrene-butadiene rubber (SBR), and acrylonitrile-butadiene rubber (NBR), and one or two or more kinds thereof may be mixed for use. Among these, so-called high cis 1,4-polybutadiene rubber containing at least 40%, preferably at least 80% of cis structure is suitably used.

[0055] The aforementioned organic peroxide is added as a cross-linker or a curing agent. Specific examples of the organic peroxide include dicumyl peroxide, 1,1-bis(t-butyl peroxy)-3,5-trimethylcyclohexane, 2,5-dimethyl-2,5-di(t-butyl peroxy)hexane, and di-t-butyl peroxide, and dicumyl peroxide is preferably used. A blended amount of the organic peroxide is preferably 0.3-2.0 parts by mass, particularly 0.3-1.0 parts by mass with respect to 100 parts by mass of the base rubber.

[0056] As α,β-unsaturated carboxylic acid blended as the co-crosslinker, α,β-unsaturated carboxylic acid having a carbon number of 3-8 such as acrylic acid or methacrylic acid can be used. Further, as metal salt of α,β-unsaturated carboxylic acid, zinc salt, magnesium salt, sodium salt, or the like thereof can be listed. Among these, zinc diacrylate which adds high resilience is preferable. The blended amount of the metal salt of α,β-unsaturated carboxylic acid is preferably 15-45 parts by mass, more preferably 25-40 parts by mass with respect to 100 parts by mass of the base rubber.

[0057] In addition to the above-described components, the rubber composition for the core can properly be blended with common additives included in the core of the golf ball, such as a specific gravity adjustment agent, an antioxidant, a plasticizer, a dispersant, an ultraviolet absorber, and a pigment as needed. As the specific gravity adjustment agent, inorganic salt such as zinc oxide, barium sulfate or calcium carbonate, metal powder having high specific gravity such as tungsten or molybdenum powder, and a mixture thereof can be used. Phenol compound can be used as the antioxidant.

EXAMPLES Examples 1-6 and Comparative Examples 1-4

[0058] (Fabrication of Core)

[0059] A core having an outside diameter of 39.0 mm was fabricated by filling a mold with a rubber composition containing 100 parts by mass of high cis butadiene rubber (BR18, produced by JSR Corporation.), 36 parts by mass of zinc diacrylate, 17.2 parts by mass of zinc oxide, 0.4 parts by mass of dicumyl peroxide, and 0.9 parts by mass of bis(pentachlorophenyl)disulfide, and vulcanizing in a mold under compression for 20 minutes at 160° C.

[0060] (Covering of Cover)

[0061] A cover composition was then prepared by mixing 25 parts by mass of Surlyn 8945 (sodium-neutralized ethylene-methacrylate copolymer ionomer) produced by Du Pont, 25 parts by mass of Surlyn 9945 (zinc-neutralized ethylene-methacrylate copolymer ionomer) produced by Du Pont, 35 parts by mass of HG252 produced by Kuraray Co., 15 parts by mass of Epofriend A1010 produced by Daicel Chemical Industries, Ltd., 2 parts by mass of titanium oxide, and 2 parts by mass of barium sulfate, and a surface of the aforementioned core was covered with the composition by an injection molding machine to form a two-piece golf ball body having a diameter of 42.8 mm.

[0062] (Formation of Coating)

[0063] PTMG650 produced by BASF as bifunctional polyol and trimethylol propane produced by Koei Chemical Co., Ltd. as trifunctional polyol were blended in ratios shown in Tables 1 and 2, diluted with toluene solvent so that polyol constitutes 30% by mass in a solid state, and added with 0.02% by weight of dibutyl tin dilaurate as a catalyst to prepare a main agent. A curing agent, on the other hand, was prepared by diluting N3500, hexamethylene diisocyanate produced by Sumika Bayer Urethane Co., Ltd., with toluene so as to constitute 60% in a solid state.

[0064] Each paint was prepared by blending the above-described main component and curing agent in an equivalent ratio of 1:1.2 and mixing for 5 minutes. The golf ball body formed as above was rotated at 150 rpm, and the paint prepared as above was sprayed on the ball body with moving an air gun upward and downward to form a coating on the surface of the ball body. A golf ball was obtained by a curing step for 120 minutes at 50° C. thereafter. Specifications of golf balls in examples 1-6 and comparative examples 1-4 are shown in Tables 1 and 2. TABLE 1 Examples 1 2 3 4 5 6 7 8 Composition BR18 100 100 100 of core zinc diacrylate 36 36 36 zinc oxide 17.2 16.7 16.2 bis(pentachlorophenyl)disulfide 0.9 0.9 0.9 dicumyl peroxide 0.4 0.4 0.4 Composition Surlyn 8945 25 of cover Surlyn 9945 25 HG252 35 Epofriend A1010 15 Composition trimethylol propane 1.5 1.5 1.5 1.5 1.2 2.3 1.5 1.5 of coating /polytetramethylene glycol Thickness of outermost layer of cover(mm):CL 1.9 1.9 1.9 1.9 1.9 1.9 1.4 0.8 Thickness of coating(μ):PL 30 50 80 120 50 50 50 50 R(PL/CL/1000) 0.016 0.026 0.042 0.063 0.026 0.026 0.036 0.063 50%-modulus(MPa) 22.3 22.3 22.3 22.3 16.2 28.8 22.3 22.3 DRY spin rate(rpm) 6700 6600 6500 6400 6700 6500 6500 6300 WET spin rate(rpm) 2950 3050 3000 2950 3050 3000 3050 3000 Spin retention(%) 44 46 46 46 46 46 47 47

[0065] TABLE 2 Comparative examples 1 2 3 4 5 Composition BR18 100 100 of core zinc diacrylate 36 36 zinc oxide 17.2 18.2 bis(pentachlorophenyl)disulfide 0.9 0.9 dicumyl peroxide 0.4 0.4 Composition Surlyn 8945 25 of cover Surlyn 9945 25 HG252 35 Epofriend A1010 15 Composition trimethylol propane 1.5 1.5 1.5 0.5 1.5 of coatin /polytetramethylene glycol Thickness of outermost Layer of cover(mm):CL 1.9 1.9 1.9 1.9 3.2 Thickness of coating(μ):PL 8 18 140 50 30 R(PL/CL/1000) 0.004 0.009 0.074 0.026 0.009 50%-modulus(MPa) 22.3 22.3 22.3 4.2 22.3 DRY spin rate(rpm) 6350 6400 7000 6800 6950 WET spin rate(rpm) 2400 2450 2750 2400 2500 Spin retention(%) 38 38 39 35 36

Examples 7, 8 and Comparative Example 5

[0066] A golf ball was fabricated as in the example 1, except for different composition of the core as shown in Tables 1 and 2.

[0067] (Evaluation Method)

[0068] Specimens and golf balls obtained with the above-described method based on specifications of the examples and comparative examples were evaluated by the following method.

[0069] (1) 50%-Modulus of Coating (MPa)

[0070] Paint prepared as above was applied on a coating glass plate and was cured. The coating formed on the glass plate was peeled to prepare a size-4 dumbbell specimen having a thickness of 0.2 mm with the coating, and a tensile test defined by JIS K6251 was performed. A tensile strength to obtain 50% elongation of the coating was measured as a 50%-modulus.

[0071] (2) Spin Retention

[0072] A sand wedge club was attached to a swing robot manufactured by True-Temper to hit a ball at a head speed of 21 m/s. The ball was accordingly shot and a mark applied to the ball were taken through sequential photography so as to measure a spin rate. The measurement was done under a normal dry condition and a wet condition generated by wetting the ball and club face with water. A spin retention is defined as a value of, spin rate under wet condition/spin rate under dry condition×100. A higher value indicates a better spin retention.

[0073] (Evaluation Result)

[0074] It is obvious from Table 1 that all the golf balls in the examples 1-8 according to the present invention show good spin retention. On the contrary, the comparative examples 1, 2, which have small coating thicknesses (PL) such as 8 μm and 18 μm and value R smaller than 0.01, and the comparative example 3, which has large coating thickness (PL) such as 140 μm, show low spin retention. The comparative example 4, which has the coating thickness (PL) of 50 μm but the 50%-modulus of the coating as low as 4.2 MPa, also shows low spin retention. In addition, the comparative example 5, which has the coating thickness (PL) of 30 μm but the thickness of the outermost layer of the cover (CL) as large as 3.2 mm and value R smaller than 0.01, also shows low spin retention.

[0075] The embodiment disclosed herein should be considered illustrative in all points, rather than limiting. The scope of the present invention is not indicated by the descriptions above, but by the appended claims. All modifications that fall in the equivalent spirit and scope of the claims are intended to be included in the present invention.

[0076] As described above, by setting the coating thickness and the 50%-modulus of the coating within prescribed ranges and setting the ratio of the thickness of the outermost layer of the cover and the coating thickness to a prescribed value, the golf ball according to the present invention can retain the high spin retention, and the coating follows deformation of the cover without peeling off the cover even for the large deformation caused by hitting the golf ball with a driver or the like.

[0077] Although the present invention has been described and illustrated in detail, it is clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation, the spirit and scope of the present invention being limited only by the terms of the appended claims. 

What is claimed is:
 1. A golf ball having a core, a cover of at least one layer covering the core and a coating formed on an outer surface of the cover, wherein a thickness of said coating is 25 to 125 μm, a 50%-modulus of the coating is 5 to 50 MPa, and a value R expressed by an equation (1) is 0.01 to 0.5: R=PL/CL/1000   (1) where CL (mm) represents a thickness of an outermost layer of the cover and PL (μm) represents a thickness of said coating.
 2. The golf ball according to claim 1, wherein the outermost layer of the cover has thickness CL (mm) of 0.5 to 2.5.
 3. The golf ball according to claim 1, wherein the coating is formed with polyurethane paint including polyol and polyisocyanate.
 4. The golf ball according to claim 3, wherein an equivalent ratio of trifunctional polyol to bifunctional polyol included in polyol (trifunctional polyolibifunctional polyol) is 1.0-2.5. 